1. Field of the Invention
The present invention relates generally to optical video disc players and, more particularly, is directed to an optical video disc player in which an optical video disc recorded in a constant linear velocity format is reproduced in a so-called scan playback mode.
2. Description of the Prior Art
In order to understand the present invention more clearly, let us first describe an example of a prior-art optical video disc player with reference to FIG. 1.
Referring to FIG. 1, there is provided a microcomputer 70 which generally controls the operation of this video disc player. An optical video disc 10 is provided, on which there is recorded an FM-modulated color composite video signal, in a predetermined format, for example, CLV (constant linear velocity) format. The signal is recorded on a spiral track on the videodisc. Due to manufacturing tolerances, or other reasons, the track may be eccentric, i.e., the radial position of the track may vary about a center of eccentricity. The optical video disc 10 is rotated by a spindle motor 51 whose rotation is servo-controlled by the spindle servo circuit 50 as will be described later.
In the reproducing circuit 20, there is provided a photo pickup head (optical head) 21 comprising a laser light emitting element, a receiving element for receiving the laser light emitted from the laser light emitting element, an objective lens, a tracking coil for moving the optical axis of the objective lens in the radial direction of the disc 10, et cetera. The photo pickup head 21 is moved in the radial direction of the optical video disc 10 by the sled motor 43.
In the tracking servo circuit 30, one portion of the output signal from the photo pickup head 21 is supplied to the detecting circuit 31 which derives a tracking error voltage Vt. This voltage Vt is supplied through an amplifier 32 to the tracking coil of the photo pickup head 21 so that the tracking of the objective lens (not shown) is servo-controlled.
In the sled servo circuit 40, the tracking error voltage Vt from the detecting circuit 31 is supplied to a low-pass filter 41 from which there is derived a DC component of the tracking error voltage Vt. This DC component is supplied through an amplifier 42 to the sled motor 43 so that a sled servo is performed.
Accordingly, in the normal reproduction mode, the tracking servo circuit 30 and the sled servo circuit 40 enable the photo pickup head 21 to correctly follow the track of the optical video disc 10 so that a reproduced signal is obtained from the photo pickup head 21.
The reproduced signal is supplied through the playback amplifier 22 and the limiter 23 to the FM demodulating circuit 24, in which it is FM-demodulated to provide a color composite video signal Sc. The color composite video signal Sc is supplied to the time base corrector (TBC) 25, in which any timebase fluctuation of the video signal Sc is removed.
The signal Sc from the FM demodulating circuit 24 is supplied to the charge coupled device (CCD) 251, and the signal Sc from the CCD 251 is supplied to the synchronizing separating circuit 252, from which there are derived a vertical synchronizing pulse PBV and a horizontal synchronizing pulse PBH2. The horizontal synchronizing pulse PBH2 is supplied to the phase comparing circuit 253. The master sync. (synchronizing) pulse generating circuit 61 generates a synchronizing pulse REFH having a reference horizontal frequency. This synchronizing pulse REFH is also supplied to the phase comparing circuit 253.
The phase comparing circuit 253 derives a phase-compared output of the pulses PBH2 and REFH. The phase-compared output is supplied to the low-pass filter 254 which derives a time base error voltage TBCE. The time base error voltage TBCE changes level in accordance with the phase difference between the pulses PBH2 and REFH. This time base error voltage TBCE is supplied to the voltage controlled oscillator (VCO) 255 as a control signal, and the oscillation signal from the VCO 255 is supplied to the CCD 252 as a clock signal.
Accordingly, the synchronizing pulse PBH2 has a constant phase synchronized with the reference pulse REFH so that any time base fluctuation in the video signal Sc from the CCD 251 caused by a jitter component, eccentricity of the track on the optical video disc 10, or the like is removed. This video signal Sc is supplied through the memory circuit 26 and the amplifier 27 to the output terminal 28.
The memory circuit 26 is utilized to effect a special playback or the like, and comprises an analog-to-digital (A/D) converter for converting the input signal Sc to a digital signal, a field memory in which the digitized signal Sc is written, a digital-to-analog (D/A) converter for converting the digital signal Sc read-out from the field memory to an analog signal and a controller for controlling the above-described circuits, though not shown. In the normal reproduction mode, the memory circuit 26 is by-passed so that the input signal Sc is directly outputted.
Further, the video signal Sc from the FM demodulating circuit 24 is supplied to the synchronizing separating circuit 52 which derives a horizontal synchronizing pulse PBH1. The horizontal synchronizing pulse PBH1 is supplied to the servo circuit 50, and the pulse REFH from the reference signal generating circuit 61 is supplied to the servo circuit 50, whereby the rotation of the spindle motor 51 is controlled so that the pulse PBH1 can be synchronized with the pulse REFH, and thus the aforementioned spindle servo is carried out.
With the employment of the spindle servo utilizing the pulses PBH1 and REFH, the lock range is narrow so that, for example, when the optical video disc 10 starts rotating or the like, the servo operation can not be properly carried out. Therefore, a frequency generator 53 is coupled to the spindle motor 51 and the output signal of the frequency generator 53 is supplied to the servo circuit 50 to back-up the spindle servo utilizing the pulses PBH1 and REFH.
The video disc player is operated in the normal playback mode as described above.
In the aforementioned video disc player, if the photo pickup head 21 is moved in the radial direction of the optical video disc 10 at a speed higher than that of the normal playback mode while the tracking of the objective lens within the photo pickup head 21 is servo-controlled, then the objective lens attempts to stay in the track by the action of the tracking servo circuit 30 against the movement of the photo pickup head 21.
When the photo pickup head 21 is moved to the controllable limit of the tracking servo, the objective lens is caused to make a track jump to a new track (in practice, at that time, the tracking servo is turned OFF and the track jump is forcibly effected) and the tracking servo is reactivated so that the photo pickup head 21 tracks the new track.
Accordingly, regardless of the movement of the photo pickup head 21 at the speed higher than that in the normal playback mode, a normal video signal Sc having no noise component due to mistracking can be intermittently produced. Therefore, utilizing the video signal Sc thus normally reproduced, it is possible to obtain a fast forward or fast rewind reproduced picture. In the following description, this operation mode will hereinafter be referred to as scan mode or scan playback mode.
In the scan playback mode, the microcomputer 70 supplies the amplifier 42 with a control signal SCN to cause the photo pickup head 21 to move towards inner or the outer periphery of the optical video disc 10 at a speed higher than that of the normal playback mode. Also, the microcomputer 70 supplies the tracking displacement amount detecting circuit 31 with a jump control signal that causes the objective lens within the photo pickup head 21 to perform a track jump.
The tracking displacement amount detecting circuit 31 detects the tracking condition by detecting the amplitude of the output video signal from the photo pickup head 21. When a jump signal Sj (which will be described later) is supplied by the microcomputer 70 to the detecting circuit 31, then the jump operation of the photo pickup head 21 becomes possible. Therefore, at that time, the aforementioned normal video signal Sc can be intermittently obtained.
The synchronizing pulses PBH2 and PBV are supplied to the memory circuit 26 as signals indicative of the timing at which the video signal Sc is supplied to the memory circuit 26. The pulse PBV is supplied to the microcomputer 70 as a signal indicative of the timing of the video signal Sc, thereby generating a write request signal WTRQ. The write request signal WTRQ is supplied to the memory circuit 26.
The sync. generating circuit 61 generates reference synchronizing pulses REFH and REFV having reference horizontal and vertical frequencies, respectively. These reference synchronizing pulses REFH and REFV are also supplied to the memory circuit 26.
As described above, when the normal video signal Sc is obtained, one field of the video signal Sc is written in the field memory of the memory circuit 26 in synchronism with the pulses PBH2 and PBV. Until the next normal video signal Sc is obtained, the video signal Sc written in the field memory is repeatedly read-out therefrom in synchronism with the pulses REFH and REFV. The thus read-out video signal Sc is supplied to the output terminal 28. Therefore, the scan playback is carried out so that a desired picture can be monitored in the fast forward or fast rewind mode, if necessary.
However, if the scan playback is carried out as described above, it causes trouble in the TBC 25, and this will be described hereinunder with reference to FIGS. 2A to 2F. FIGS. 2A to 2F are timing charts to which reference will be made in explaining the present invention, respectively.
As shown by the time period before the time point t1 in FIGS. 2A to 2C, in the normal playback mode, the reference synchronizing pulse REFH and the reproduced synchronizing pulse PBH1 are substantially the same in phase. Accordingly, as shown by the time period before the time t.sub.1 in FIG. 2E, the time base error voltage TBCE more or less fluctuates around a central value Ec thereof in response to a fluctuation of the time base.
If a track jump is carried out during the time between the time t.sub.1 and the time t.sub.2 in order to effect a scan playback, then the pulses PBH1 and PBH2 derived from the sync. separating circuit 252 become noise components during the period between the times t.sub.1 and t.sub.2. In practice, the jitter component is compensated for by the CCD 251 so that the pulse PBH2 is delayed in phase from the pulse PBH1 by a predetermined delay time. This is negligible in the following description. Further, the microcomputer 70 controls the low-pass filter 254 so as to hold the value of the time base error voltage TBCE at time point t.sub.1 from the time point t.sub.1.
If the track jump ends at the time t.sub.2 and the tracking is stabilized at the time t.sub.3, then the normal pulses PBH1 and PBH2 are again obtained from the next time point t.sub.4. The durations of the periods of t.sub.1 to t.sub.2 and t.sub.2 to t.sub.3 are, for example, 8 milliseconds and 2 milliseconds, respectively, and the time t.sub.4 is the time at which the first pulse PBH1 is generated after the time point t.sub.3. Accordingly, as compared with the duration of t.sub.1 to t.sub.3, t.sub.3 .apprxeq.t.sub.4 is satisfied.
The time t.sub.4 at which the pulse PBH1 is obtained is random relative to the time point whereat the reference synchronizing pulse REFH is obtained, and these two time points are generally not coincident with each other as shown in FIGS. 2A and 2B. If the time points of the two pulses PBH1 and REFH are not coincident, then the TBC 25 must compensate for the time base fluctuation while additionally absorbing the phase difference between the two pulses PBH1 and REFH. Consequently, a wide compensation range is required by the TBC 25.
Therefore, when the pulse PBH1 is obtained at the time t.sub.4, a control signal HSFT from the microcomputer 70 is supplied to the sync. generating circuit 61, whereby as shown in FIG. 2D the pulse REFH' is reset at the time t.sub.4 and is made the same in phase as the pulse PBH1. Thereafter, the pulse REFH' is generated at every horizontal period, and the hold on the TBC 25 is released from the time point t.sub.4.
Accordingly, after the time t.sub.4, the TBC 25 compensates for the time base fluctuation similarly to the time period before the time t.sub.1 so that, when the next vertical synchronizing pulse PBV is obtained, a video signal Sc of one field succeeding to the pulse PBV is written in the memory circuit 26. In practice, it is frequently observed that the time t.sub.2 must be delayed until the response of the spindle servo circuit 50 becomes satisfactory after the track jump. For the sake of simplicity, this will not be described herein.
The track on the optical video disc 10 is eccentric so that, while the phase .theta..sub.REF of the reference horizontal synchronizing pulse REFH is constant as shown in FIGS. 3A and 3B, the phase .theta..sub.PBH of the reproduced horizontal synchronizing pulse PBH1 fluctuates around the phase .theta..sub.REF and has a period coinciding with the rotation of the optical video disc 10. In the rotation cycle of the CLV disc 10, one rotation corresponds with a period of 2 fields in the innermost periphery of the disc 10 and about 5.3 fields in the outermost periphery of the disc 10.
In the scan playback mode, the pulse REFH is reset so that it coincides in phase with the pulse PBH1 at the time t.sub.4 as described above. Assuming now that this reset time point is the time point at which the delay of the phase .theta..sub.PBH of the pulse PBH1 becomes maximum as shown by a cross mark in FIG. 3A, then the TBC 25 must become capable of compensating for the time base of the output video signal Sc in the advanced direction by the peak-to-peak value .DELTA..theta. in the range in which the phase .theta..sub.PBH of the reproduced pulse PBH1 is changed.
Conversely, assuming that the reset time t.sub.4 is the time at which the advance of the phase .theta..sub.PBH of the reproduced pulse PBH1 is maximized as shown by a solid circle in FIG. 3A, then the TBC 25 must become capable of compensating for the time base of the output video signal Sc in the delayed direction by a value .DELTA..theta.. Therefore, because the track on the optical video disc 10 is eccentric, the TBC 25 needs a time base compensation range of 2.DELTA..theta. as shown in FIG. 3C, which unavoidably increases the number of coupling stages of the CCD 251.
Alternatively, as shown in FIG. 2E, if the time base compensation range of the TBC 25 for the eccentricity of the track on the optical video disc 10 is smaller than the value of 2.DELTA..theta., the time base error voltage TBCE of the TBC 25 is fixed to a maximum value Eu or minimum value Ed of the error output, whereby the TBC 25 can not operate normally, and thus a disturbed picture is reproduced.